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SCANF(3) Linux Programmer's Manual SCANF(3)
scanf, fscanf, sscanf, vscanf, vsscanf, vfscanf - input format conversion
#include <stdio.h>
int scanf(const char *format, ...);
int fscanf(FILE *stream, const char *format, ...);
int sscanf(const char *str, const char *format, ...);
#include <stdarg.h>
int vscanf(const char *format, va_list ap);
int vsscanf(const char *str, const char *format, va_list ap);
int vfscanf(FILE *stream, const char *format, va_list ap);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
vscanf(), vsscanf(), vfscanf():
_XOPEN_SOURCE >= 600 || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;
or cc -std=c99
The scanf() family of functions scans input according to format as described
below. This format may contain conversion specifications; the results from
such conversions, if any, are stored in the locations pointed to by the
pointer arguments that follow format. Each pointer argument must be of a type
that is appropriate for the value returned by the corresponding conversion
specification.
If the number of conversion specifications in format exceeds the number of
pointer arguments, the results are undefined. If the number of pointer
arguments exceeds the number of conversion specifications, then the excess
pointer arguments are evaluated, but are otherwise ignored.
The scanf() function reads input from the standard input stream stdin,
fscanf() reads input from the stream pointer stream, and sscanf() reads its
input from the character string pointed to by str.
The vfscanf() function is analogous to vfprintf(3) and reads input from the
stream pointer stream using a variable argument list of pointers (see
stdarg(3). The vscanf() function scans a variable argument list from the
standard input and the vsscanf() function scans it from a string; these are
analogous to the vprintf(3) and vsprintf(3) functions respectively.
The format string consists of a sequence of directives which describe how to
process the sequence of input characters. If processing of a directive fails,
no further input is read, and scanf() returns. A "failure" can be either of
the following: input failure, meaning that input characters were unavailable,
or matching failure, meaning that the input was inappropriate (see below).
A directive is one of the following:
o A sequence of white-space characters (space, tab, newline, etc.; see
isspace(3)). This directive matches any amount of white space,
including none, in the input.
o An ordinary character (i.e., one other than white space or '%'). This
character must exactly match the next character of input.
o A conversion specification, which commences with a '%' (percent)
character. A sequence of characters from the input is converted
according to this specification, and the result is placed in the
corresponding pointer argument. If the next item of input does not
match the conversion specification, the conversion fails -- this is a
matching failure.
Each conversion specification in format begins with either the character '%'
or the character sequence "%n$" (see below for the distinction) followed by:
o An optional '*' assignment-suppression character: scanf() reads input
as directed by the conversion specification, but discards the input.
No corresponding pointer argument is required, and this specification
is not included in the count of successful assignments returned by
scanf().
o An optional 'a' character. This is used with string conversions, and
relieves the caller of the need to allocate a corresponding buffer to
hold the input: instead, scanf() allocates a buffer of sufficient size,
and assigns the address of this buffer to the corresponding pointer
argument, which should be a pointer to a char * variable (this variable
does not need to be initialized before the call). The caller should
subsequently free(3) this buffer when it is no longer required. This
is a GNU extension; C99 employs the 'a' character as a conversion
specifier (and it can also be used as such in the GNU implementation).
o An optional decimal integer which specifies the maximum field width.
Reading of characters stops either when this maximum is reached or when
a nonmatching character is found, whichever happens first. Most
conversions discard initial white space characters (the exceptions are
noted below), and these discarded characters don't count toward the
maximum field width. String input conversions store a null terminator
('\0') to mark the end of the input; the maximum field width does not
include this terminator.
o An optional type modifier character. For example, the l type modifier
is used with integer conversions such as %d to specify that the
corresponding pointer argument refers to a long int rather than a
pointer to an int.
o A conversion specifier that specifies the type of input conversion to
be performed.
The conversion specifications in format are of two forms, either beginning
with '%' or beginning with "%n$". The two forms should not be mixed in the
same format string, except that a string containing "%n$" specifications can
include %% and %*. If format contains '%' specifications then these
correspond in order with successive pointer arguments. In the "%n$" form
(which is specified in POSIX.1-2001, but not C99), n is a decimal integer that
specifies that the converted input should be placed in the location referred
to by the n-th pointer argument following format.
The following type modifier characters can appear in a conversion
specification:
h Indicates that the conversion will be one of d, i, o, u, x, X, or n and
the next pointer is a pointer to a short int or unsigned short int
(rather than int).
hh As for h, but the next pointer is a pointer to a signed char or
unsigned char.
j As for h, but the next pointer is a pointer to an intmax_t or a
uintmax_t. This modifier was introduced in C99.
l Indicates either that the conversion will be one of d, i, o, u, x, X,
or n and the next pointer is a pointer to a long int or unsigned long
int (rather than int), or that the conversion will be one of e, f, or g
and the next pointer is a pointer to double (rather than float).
Specifying two l characters is equivalent to L. If used with %c or %s
the corresponding parameter is considered as a pointer to a wide
character or wide-character string respectively.
L Indicates that the conversion will be either e, f, or g and the next
pointer is a pointer to long double or the conversion will be d, i, o,
u, or x and the next pointer is a pointer to long long.
q equivalent to L. This specifier does not exist in ANSI C.
t As for h, but the next pointer is a pointer to a ptrdiff_t. This
modifier was introduced in C99.
z As for h, but the next pointer is a pointer to a size_t. This modifier
was introduced in C99.
The following conversion specifiers are available:
% Matches a literal '%'. That is, %% in the format string matches a
single input '%' character. No conversion is done (but initial white
space characters are discarded), and assignment does not occur.
d Matches an optionally signed decimal integer; the next pointer must be
a pointer to int.
D Equivalent to ld; this exists only for backward compatibility. (Note:
thus only in libc4. In libc5 and glibc the %D is silently ignored,
causing old programs to fail mysteriously.)
i Matches an optionally signed integer; the next pointer must be a
pointer to int. The integer is read in base 16 if it begins with 0x or
0X, in base 8 if it begins with 0, and in base 10 otherwise. Only
characters that correspond to the base are used.
o Matches an unsigned octal integer; the next pointer must be a pointer
to unsigned int.
u Matches an unsigned decimal integer; the next pointer must be a pointer
to unsigned int.
x Matches an unsigned hexadecimal integer; the next pointer must be a
pointer to unsigned int.
X Equivalent to x.
f Matches an optionally signed floating-point number; the next pointer
must be a pointer to float.
e Equivalent to f.
g Equivalent to f.
E Equivalent to f.
a (C99) Equivalent to f.
s Matches a sequence of non-white-space characters; the next pointer must
be a pointer to character array that is long enough to hold the input
sequence and the terminating null character ('\0'), which is added
automatically. The input string stops at white space or at the maximum
field width, whichever occurs first.
c Matches a sequence of characters whose length is specified by the
maximum field width (default 1); the next pointer must be a pointer to
char, and there must be enough room for all the characters (no
terminating null byte is added). The usual skip of leading white space
is suppressed. To skip white space first, use an explicit space in the
format.
[ Matches a nonempty sequence of characters from the specified set of
accepted characters; the next pointer must be a pointer to char, and
there must be enough room for all the characters in the string, plus a
terminating null byte. The usual skip of leading white space is
suppressed. The string is to be made up of characters in (or not in) a
particular set; the set is defined by the characters between the open
bracket [ character and a close bracket ] character. The set excludes
those characters if the first character after the open bracket is a
circumflex (^). To include a close bracket in the set, make it the
first character after the open bracket or the circumflex; any other
position will end the set. The hyphen character - is also special;
when placed between two other characters, it adds all intervening
characters to the set. To include a hyphen, make it the last character
before the final close bracket. For instance, [^]0-9-] means the set
"everything except close bracket, zero through nine, and hyphen". The
string ends with the appearance of a character not in the (or, with a
circumflex, in) set or when the field width runs out.
p Matches a pointer value (as printed by %p in printf(3); the next
pointer must be a pointer to a pointer to void.
n Nothing is expected; instead, the number of characters consumed thus
far from the input is stored through the next pointer, which must be a
pointer to int. This is not a conversion, although it can be
suppressed with the * assignment-suppression character. The C standard
says: "Execution of a %n directive does not increment the assignment
count returned at the completion of execution" but the Corrigendum
seems to contradict this. Probably it is wise not to make any
assumptions on the effect of %n conversions on the return value.
These functions return the number of input items successfully matched and
assigned, which can be fewer than provided for, or even zero in the event of
an early matching failure.
The value EOF is returned if the end of input is reached before either the
first successful conversion or a matching failure occurs. EOF is also
returned if a read error occurs, in which case the error indicator for the
stream (see ferror(3)) is set, and errno is set indicate the error.
EAGAIN The file descriptor underlying stream is marked nonblocking, and the
read operation would block.
EBADF The file descriptor underlying stream is invalid, or not open for
reading.
EILSEQ Input byte sequence does not form a valid character.
EINTR The read operation was interrupted by a signal; see signal(7).
EINVAL Not enough arguments; or format is NULL.
ENOMEM Out of memory.
ERANGE The result of an integer conversion would exceed the size that can be
stored in the corresponding integer type.
The functions fscanf(), scanf(), and sscanf() conform to C89 and C99 and
POSIX.1-2001. These standards do not specify the ERANGE error.
The q specifier is the 4.4BSD notation for long long, while ll or the usage of
L in integer conversions is the GNU notation.
The Linux version of these functions is based on the GNU libio library. Take
a look at the info documentation of GNU libc (glibc-1.08) for a more concise
description.
The GNU C library supports a nonstandard extension that causes the library to
dynamically allocate a string of sufficient size for input strings for the %s
and %a[range] conversion specifiers. To make use of this feature, specify a
as a length modifier (thus %as or %a[range]). The caller must free(3) the
returned string, as in the following example:
char *p;
int n;
errno = 0;
n = scanf("%a[a-z]", &p);
if (n == 1) {
printf("read: %s\n", p);
free(p);
} else if (errno != 0) {
perror("scanf");
} else {
fprintf(stderr, "No matching characters\n"):
}
As shown in the above example, it is only necessary to call free(3) if the
scanf() call successfully read a string.
The a modifier is not available if the program is compiled with gcc -std=c99
or gcc -D_ISOC99_SOURCE (unless _GNU_SOURCE is also specified), in which case
the a is interpreted as a specifier for floating-point numbers (see above).
Since version 2.7, glibc also provides the m modifier for the same purpose as
the a modifier. The m modifier has the following advantages:
* It may also be applied to %c conversion specifiers (e.g., %3mc).
* It avoids ambiguity with respect to the %a floating-point conversion
specifier (and is unaffected by gcc -std=c99 etc.)
* It is specified in the upcoming revision of the POSIX.1 standard.
All functions are fully C89 conformant, but provide the additional specifiers
q and a as well as an additional behavior of the L and l specifiers. The
latter may be considered to be a bug, as it changes the behavior of specifiers
defined in C89.
Some combinations of the type modifiers and conversion specifiers defined by
ANSI C do not make sense (e.g. %Ld). While they may have a well-defined
behavior on Linux, this need not to be so on other architectures. Therefore
it usually is better to use modifiers that are not defined by ANSI C at all,
that is, use q instead of L in combination with d, i, o, u, x, and X
conversions or ll.
The usage of q is not the same as on 4.4BSD, as it may be used in float
conversions equivalently to L.
getc(3), printf(3), setlocale(3), strtod(3), strtol(3), strtoul(3)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
GNU 2010-09-20 SCANF(3)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface